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Testing & Troubleshooting of
Fibre Optic Cables
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• To be discussed
– Fundamental concepts of optical measurement– Standard fibre optic cable tests– Standard fibre optic equipment tests– Data transmission tests– Laboratory measurements
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Fundamental concepts of optical measurement
• Optical power– Measured in watts– Optical measurements relate directly to electrical
measurements – Optical energy measured as average of incoming
optical pulsed signal
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Power for received optical waveform
POWER(dBm)
TIME
Peak
Average
-16
-21-23
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– Light constructed of photons– The higher the frequency the more energy in the photon– Light energy is directly proportional to frequency and
inversely proportional to wavelength– C = x f– Planck’s law– Q = h x f
• Q = energy of photon• h = Planck’s constant
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• Electrical power– P = dQ / dt
Q = electron energy in joules
• Light power– P = d (nQp) / dt
Qp = energy of a single photonn = number of photons
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• Power measurement
– Different materials used for different wavelengths• e.g: Silicon reacts strongly at 850 m • Gallium arsenide reacts strongly at 1300 & 1550 m
– Therefore instruments are calibrated for the different wavelengths
– Measure average power
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• Bandwidth
– 3 dB limits as discussed– For optical detector a 3 dB drop in optical level
represents a 6 dB drop in electrical output.– Power meter shows correct optical power
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Fibre optic tests• Generally tests carried out before and after
installation• Transmitter power test
• Connect Tx device via 2 m fibre to power meter• Should be +/- 5% of specified figure
• Receiver performance test• Connect Tx to Rx through optical fibre & attenuators.
Drop Rx level down to specified receiver sensitivity• Check BER is < 10-9
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Measuring power into fibre
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Testing the Receiver
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• Continuity testing
– Very basic test of fibre continuity– A continuity tester works at a 650m
wavelength (red).– Use physical observation – Used for
• Tracing cores• Finding fractures or bad splices with leaking
light• Locating cores at intermediate points by
bending the fibres
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• Insertion loss testing– The most commonly used test– Firstly calibrate the power meter to the optical
source• Use a 2 metre fibre• Generally to -10 dBm
– Check• Calibration fibre is the same type as the installed fibre• Test wavelength is the same as the installed
operating wavelength• Test source is the same as the installed source (led or
laser)» Cont
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• Test connectors are the same as the installed connectors
– Cable insertion loss is calculated as • Power source output value minus power meter
reading– Carry out the test in both directions (diameter
and NA mismatches)– Perform on every fibre in cable– Where possible use the actual transmitter as a
source on some cores to compare the results
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Insertion loss measurement
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• Optical time domain reflectometry (OTDR)
– A short pulse is injected into the fibre– The strength of the reflection shows attenuation– The time delay in the return of the reflection shows
distance to the fault– Reflections can be caused by
• Connectors, cracks, splices,impurities, breaks• Continual reflection from backscatter (rayleigh scattering)
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Trace from Trace from an OTDR OTDR
4 dB/Div
2km/Div
DeadBand
L
L
L
L
5 4 2
5
5
6 1
7
3
L = Loss introduced by inline obstruction
Distance
Attenuation
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– Accuracies of 1m and 0.01dB loss are achievable
– Carry out tests on the reel and after installation– This is a relative measurement therefore
should not be used for insertion loss measurements.
– Carry out in both directions (different diameters and NAs)
– For helical fibre laid cables use the fibre / cable length ratio to calculate distance to fault
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• Bit Error Rate testing
– BER = error bits received total bits sent for a given period of time
– Normally 30 mins to 1 hr– Expect residual BERof 10-9
– Availability• Time that link is available for uninterrupted use
over a 12 month period at a BER of 10-12
• Expect 99.9999% or better
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BER vs RSL for optical system
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• Eye diagrams
– Provides analysis of the communications system in real time
– Formed by continually overlapping the signal at the Rx input on a scope (with a pseudo random signal from the Tx)
– Measures• Noise, jitter, period, duty cycle, settling time,
pulse width, rise time, fall time, overshoot, undershoot, preshoot, settling time, pulse quality and extinction ratio.
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Eye diagram
Logic 1 overshoot
Logic 0 overshoot
Logic 1 undershoot
Logic 0 undershoot
Eye Width
10%
90%
EyeOpeningHeight
RiseTime
FallTime
Noise and Distortion
Jitter
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Other tests• Laboratory tests
– Wavelength measurement– Dispersion measurement– Bandwidth measurement– Phase measurement– Polarisation measurement– Number of travelling modes– NA measurement– Core diameter
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Thank You For Your Interest
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